In retail stores, such as disk selling stores, an antitheft device is used to protect articles, such as CD records, from shoplifting.
Where the article is a CD record, for example, the antitheft device comprises a body unit incorporating a battery-powered buzzer and a receiving circuit, with transparent acrylic plates attached to the body unit. The CD record, case-packed, is inserted into a storage space defined by the body unit and acrylic plates.
CD records, in such condition, are displayed in a disk store, and a customer who wants to buy such a CD record takes the CD record, with the body unit attached thereto, to the counter. Then, a clerk makes the body unit inoperative to sound an alarm by using a tool before the CD record is taken out from the storage space and handed to the customer.
If the CD record is taken out from the storage space without using the tool, the body unit operates to sounds an alarm. If an attempt is made to smuggle the CD record out of the store without having it taken out from the storage space, or with the body unit attached to the CD record, the receiving circuit senses a signal radiated from the transmitter exclusively to a particular area adjacent the doorway of the store to cause the buzzer to sound an alarm.
In known antitheft devices of this type, electromagnetic waves of VHF or UHF band are generally used for transmission of signals from the transmitter to the body unit.
However, the use of electromagnetic waves of such high frequency band presents several problems as mentioned below.
(1) The tuned frequency at the receiving circuit is likely to fluctuate, and because of large fluctuations involved, the receiving circuit has only limited allowance for frequency fluctuation of signals received. PA0 (2) Misalarming is likely to occur due to extraneous radiation. PA0 (3) Malfunction is likely to be caused to other equipment. PA0 (4) There must be provided compensator means for eliminating problems (1) to (3) above, and because of the presence of the compensator means, the receiver portion of the body unit is necessitated to be of a large size.